The base stations of cellular telecommunications networks are conventionally located adjacent to the antenna in a casing at the base of the antenna tower. The footprint required for such a casing can make it challenging to find suitable sites for base stations. For example, if a base station is to be mounted on a rooftop, structural reinforcement may be required. Further, conventional base stations require both primary and back-up power sources.
The Common Public Radio Interface (CPRI) Specifications are available from http://www.cpri.info/spec.html (the current version of which is V4.1, which is hereby fully incorporated by reference), specifies an interface between the baseband Radio Equipment Control (REC) and Radio Equipment (RE) in a wireless base station, commonly known as a “remote radio head”. CPRI allows inter-operability of equipment from different vendors. The CPRI interface is also known as “interface C”.
CPRI facilitates a distributed architecture where base stations, which contain the REC, are connected to remote radio heads, containing the RE, via fibre optic links that carry the CPRI data. Such an architecture can reduce costs because only the remote radio heads containing the RE need to be situated in challenging locations, whereas the REC component can be located in a different position, where footprint, environment and availability of power may be more easily catered for.
The CPRI protocol is based on the basic 3G UMTS base station (node B) 10 ms radio frame, and has evolved from the original specification to include provisions for support of LTE, WCDMA, WiMAX-based systems, and other Standards.
As shown in FIG. 1, each 10 ms base station frame comprises 150 hyperframes, each having a length of about 66.67 microseconds. The hyperframes comprise 256 basic frames, each having a length of about 260 nanoseconds. Each basic frame comprises sixteen words of sixteen bits. The first word contains control data. The subsequent fifteen words contain user play In Phase/Quadrature-Phase (IQ) data.
The basic frame is the basic timing unit of the CPRI protocol. As mentioned above, each basic frame is about 260 nanoseconds in duration, which gives a frame rate of 3.84 Mbps/MHz. This is also the UMTS chip rate, and the period corresponds to 1 UMTS chip. The chip rate is the number of pulses per second.
The most recent versions of the CPRI Specification provides methods of mapping UMTS, LTE and WiMAX in symbol data to the basic frame rate of 3.84 MHz. However, these mapping schemes are not suitable for GSM, and, if applied to GSM, would add quite some delay to GSM transmission over the air interface, and require similar buffering at the remote radio head.